Incineration of lime-conditioned sewage sludge with high sulfur fuel

ABSTRACT

Sewage sludge is dewatered with the aid of lime and then incinerated using high sulfur fuel. During incineration the lime reacts with the sulfur in the fuel and with oxygen to form the solid CaSO 4  for disposal and thus prevent formation of the pollutant, SO 2  gas. High sulfur coal may be crushed and used as an additional filter aid or sludge conditioner and the serve as the auxiliary fuel during incineration.

This invention is directed to a non-polluting process for incinerationof sewage sludge using a high sulfur fuel.

The purpose of a sewage sludge incineration process is to completelyconsume the organic matter in the sludge, leaving as a residue only aninert ash, and to perform this combustion reaction in a manner that doesnot produce noxious gases or objectionable odors. In carrying out thisincinerator process, it is generally necessary to use some auxiliaryfuel. Either natural gas or No. 2 fuel oil is the most common fuel usedtoday for this purpose.

A dewatering procedure is normally carried out on the sludge produced asthickener underflow, to prepare the sludge for incineration. Thisdewatering is carried out in filters and the sludge is usuallyconditioned for dewatering by the use of organic polymers or inorganicchemicals to improve the dewatering properties of the sludge. Naturalgas and No. 2 fuel oil are no longer plentiful and are thereforeexpensive at present and will likely become more expensive in thefuture. From an economic and/or availability standpoint it would be veryadvantageous to use a high sulfur coal or a high sulfur residual oil(Bunker C, No. 6, for example), containing 1/2% or more, by weight,sulfur, as auxiliary fuel. The use of these high sulfur fuels would,however, result in pollution of the atmosphere with SO₂ being emittedwith the incinerator stack gases.

Accordingly, it is an object of the present invention to provide amethod for dewatering and incinerating sewage sludge using high sulfurfuel in which a dewatering aid reacts during incineration to eliminatepolluting SO₂ gases from the incinerator exhaust gases.

Other objects and advantages will become apparent from the followingdescription taken in conjunction with the accompanying drawings inwhich:

FIG. 1 is a flow diagram of the process in accordance with thisinvention and

FIG. 2 is a view partially in section of a fludized bed reactor suitablefor use in the process of the invention.

In accordance with the present invention lime (CaO) is used as theinorganic chemical for conditioning the sludge ahead of finaldewatering. The residual lime present in the dewatered sludge feed tothe incinerator reacts with the sulfur in the auxiliary fuel and withoxygen to form calcium sulfate (CaSO₄), eliminating any air pollution bySO₂ in the incinerator stack gases. It is generally convenient to addsufficient lime in the dewatering step to satisfy the lime requirementof the lime-sulfur reaction of the incineration step.

In addition, where the high sulfur fuel is coal, the coal may be crushedto a fine particle size, say -28 mesh to -150 mesh and employed as afilter aid or sludge conditioner in the final dewatering of the sludge.This fine coal is thus present in the filter cake and serves as fuel inthe incineration step.

It should be noted that the raw sewage waste stream may also containsulfur which must be neutralized in the incineration step.

Referring now to FIG. 1, the process of the invention is illustrated inthe form of a flow diagram and operates upon the underflow of athickening stage 60 and proceeds through a dewatering step 70 to thefinal incineration step 80. The raw sewage waste stream is introducedinto the thickening stage through line 59. The overflow 61 of thethickening stage is removed from the system and disposed of by means notof concern in this process. The thickener underflow moves to thedewatering step 70 through line 62. Lime (CaO) is added to the flow inline 62 through line 63. The dewatering stage is conducted using filterpresses or vacuum drum filters. Filter presses are preferred since theyproduce a dryer filter cake. The liquid separated in the dewatering stepis conducted out of the process through line 71 for separate disposal.The sewage sludge product or filter cake is conducted to theincineration step which may be carried out in a fluidized bed reactor orin other types of incinerators; for example, the multiple hearthfurnace. Air is introduced into the incinerator through line 81 and ahigh sulfur fuel is introduced through line 82. Combustion occurs in theincinerator at temperatures in the range from about 1300° F. to 1600° F.so that the organic matter in the sewage sludge is completely consumedand destroyed and a reaction between the sulfur of the fuel and the limeaddition occurs as follows:

    CaO + S + 1.5 O.sub.2 → CaSO.sub.4.

in general, 50% excess CaO is provided over the theoretical CaOrequirement for sulfur neutralization to assure that essentially all thesulfur is reacted.

The combustion gases are removed from the incineration step through line84 and the solids, including ash and CaSO₄ are removed through line 85.

Referring now to FIG. 2, a fluidized bed incinerator 20 is shown as onetype of reactor in which the process of the invention can be carriedout. The fluidized bed reactor 20 comprises an outer shell 10 which iscapped by a roof 12 in which an exhaust gas conduit 15 is provided.Conical wall 18 forms the bottom of reactor 20 and is provided withclean-out port 19. An air inlet pipe 23 is provided through whichfluidizing gas is supplied to reactor 20. Within shell 10 is ahorizontal constriction plate 27, having tuyeres 29 therein, whichdivides the reactor 20 into a reaction chamber 31 above the constrictionplate 27 and a windbox 33 therebelow. The constriction plate 27 iscapable of supporting a fluidized bed 35 thereon. A screw feed mechanism37 is provided to feed dewatered sewage sludge or filter cake intoreaction chamber 31. Auxiliary fuel (coal or oil, for example) may beintroduced through fuel guns 39 and above-bed air may be introduced intoreaction chamber 31, if desired, by means of conduits 41. In phantom, ameans for providing a hot windbox is illustrated. Thus, exhaust gasesfrom conduit 15 is routed through the heat exchanger 48. Combustion airis heated in heat exchanger 48 and forwarded to windbox 33 through lines49 and 23. Preheated combustion air effects a saving in fuel. It will beunderstood that the process of the invention may be as well conducted inother types of incinerators, including multiple hearth furnaces.

For the purpose of giving those skilled in the art a betterunderstanding of the invention, the following examples are offered

Example I

A thickener underflow containing 5% of dry solids is subjected tofiltration in a filter press with CaO added as a filtration aid. Thesewage filter cake is then incinerated in a hot windbox fluidized bedincinerator using coal containing 4% sulfur. The amount of CaO added tothe thickener underflow is sufficient to satisfy the requirement for afiltration aid as well as the requirement for reaction with the sulfurcontained in the coal and in the sludge, taking into consideration thefact that some of the lime is removed from the process in solution withthe filtrate. Thus, where 16% dry solids are present in the filter cakefeed to the incinerator, the required CaO is determined as follows per100 pounds of dry sewage sludge solids:

    ______________________________________                                        Pounds CaO for sludge conditioning                                                                       10                                                 Pounds CaO dissolved in filtrate water                                                                   2.2                                                Pounds coal required       66                                                 Pounds sulfur in coal      2.6                                                Pounds sulfur in sewage sludge                                                                           1.0                                                Total sulfur (pounds)      3.6                                                Theoretical CaO requirement for                                               sulfur (pounds)            6.3                                                Pounds CaO for 50% excess CaO                                                                            9.45                                               Pounds total CaO required  11.65                                              ______________________________________                                    

With the addition of 11.65 pounds of CaO per 100 pounds of dry sewagesludge solids, the filtration is quite effective and in the subsequentincineration the amount of SO₂ present in the stack gas is at anacceptably low level; i.e., about 10 ppm.

Example II

A thickener underflow similar to that of Example I is dewatered andincinerated in a cold windbox fluidized bed incinerator using coalcontaining 2% sulfur. With 20% dry solids present in the filter cakefeed to the incinerator, the required CaO per 100 pounds of dry sewagesludge is determined.

    ______________________________________                                        Pounds CaO for sludge conditioning                                                                      10                                                  Pounds CaO dissolved in filtrate                                                                        2.5                                                 Pounds coal required      87                                                  Pounds sulfur in coal     1.7                                                 Pounds sulfur in sewage sludge (1%)                                                                     1.0                                                 Total sulfur (pounds)     2.7                                                 Theoretical CaO required for sulfur                                                                     4.7                                                 Pounds CaO for sulfur at 50% excess                                                                     7.05                                                Pounds total CaO required 9.55                                                ______________________________________                                    

In this example, the amount of CaO added as filtration aid satisfies therequirement for reaction with the sulfur. Again, the filtration iseffective and the SO₂ present in the stack gas is less than 10 ppm.

Although the present invention has been described with particularreference to preferred embodiments, it will be apparent to those skilledin the art that variations and modifications may be made withoutdeparting from the essential spirit and scope of the invention. It isintended to include all such variations and modifications.

I claim:
 1. An economical, non-polluting process for the disposal of asewage waste stream in which the waste stream is dewatered and thesludge formed thereby is incinerated, comprising the steps of;a. addinga quantity of lime to the sewage waste stream sufficient to function asa filtering aid, b. dewatering the sewage waste stream by filtering toform sludge filter cake containing a relatively large amount of limetherein, c. incinerating the sludge filter cake using a high-sulfur fuelas auxiliary fuel at a temperature at which the sulfur present in thefuel and in the sludge filter cake reacts with the lime in the sludgefilter cake and with oxygen to produce CaSO₄ and d. removing the CaSO₄with the solids produced by the incineration operation for disposal. 2.The process of claim 1 wherein the quantity of lime added to said sewagewaste stream is at least 10 pounds per 100 pounds of dry solids in saidfilter cake.
 3. The process of claim 1 wherein the quantity of limepresent in said filter cake is 50% in excess of the theoreticalrequirment for reaction with the sulfur present during incineration. 4.The process of claim 1 wherein the auxiliary fuel is a high-sulfur oilcontaining 1/2%,, by weight, or more of sulfur.
 5. The process of claim1 wherein the auxiliary fuel is a high-sulfur coal containing 1/2%, byweight, or more of sulfur.
 6. The process of claim 1 wherein theincineration is conducted as a fluidized bed reaction.
 7. The process ofclaim 6 wherein the incineration is carried out in the temperature rangefrom about 1200° F. to about 1600° F.
 8. The process of claim 1 whereinthe dewatering step is a filter press operation.
 9. An economical,non-polluting process for the disposal of a sewage waste stream in whichthe waste stream is dewatered and the sludge produced thereby isincinerated, comprising the steps of;a. adding lime to said sewage wastestream as a filtering aid in the amount of at least 10 pounds per 100pounds of dry solids in the filter cake produced in the dewatering stephereafter, b. dewatering the sewage waste stream in a filter press toform sludge filter cake containing lime in an amount at least 50% inexcess of that required for reaction with the sulfur present in theincineration step hereafter, c. incinerating the sludge filter cake in afluidized bed reactor at a temperature from about 1200° F. up to about1600° F. using a high-sulfur auxiliary fuel whereby the sulfur presentduring incineration reacts with the lime in the filter cake and withoxygen to produce CaSO₄ and d. removing the CaSO₄ with the other solids,including ash, produced during incineration.